Well logging process



Oct. 27, l1959 A. T. sAYRE, JR

WELL LOGGING PROCESS 2 Sheets-Sheet 1 Filed Dec. 27, 1956 COM NGI

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WELL LOGGING PRocEss 2 Sheets-Sheet 2 Filed Dec. 2'?,` 1956 QUIQZ` Z:WNDU LQ Mmmm ,20ml WOZQLQQ use@ QZC. Z300 (M9) SLV! SNLLNI'IOO SSVHSAVvdi ATTORNEY `formation conditions.

ite States `Patent Patented Oct. 27, 1959 dice WELL LOGGING PROCESSAllyn T. Sayreyr., Fort Morgan, Colo., assignor to The Pure Dil Company,Chicago, Ill., a corporation of @hic Application llecemher Z7, 1956,Serial No. 630,968- 8 Claims. (Cl. Z50-43.5)

rIhis `invention `relates to a subsurfacegeologic survey technique. Itis more directly concerned with Yformation 1logging by analyzingrepresentative -geological sections `interstitial water content,core-water-salinity and others.

Because of the need for utilizing a drilling mud for obtaining coresamples by means of conventional .core drilling techniques, certainlimitations are imposed upon core analysis because of the'ushing of thecore by the drilling mud filtrate. As a result of the ushing, theconstitution and condition of the core as analyzed will be changed fromthe original reservoir conditions. When water-base muds are employed,the iltratelossiswaten oil is generally hushed from the core, and theoil and water saturation values as determined 4from core analysis `arenot representative of conditions existing -inthe reservoir. Similarly,when oil-base mud is employedinthe coring operation the filtrate loss isoil and Y'the .oil and `water-saturation values can alsobe aiected.Because of this invasion of the core by the mud iltrate during thecoring operation, it is essentialto `know the degree to which theresults of the-core analysis-dilierfrom actual Unless `the extent ofthis contamination is known reliable determinations-of lthe originalHuid saturations in the core are impossible. It is,

therefore, an object of this invention to provide a wellcoring techniquewhich permits the rapid determination of the depth of mudtiltrateinvasion of the core sample. Another object of this invention isto provide a nondestructive test for determining the effect ofthefiu'shing action of the drilling mud filtrate emplyedin obtaining thecore on the natural tluid content of recovered core samples. lt is anadditional object of this invention to provide a radioactivechemical-tracer method fordetermining the degree to which the mudfiltrate resulting from the use of a drilling,r mud in the Well-coringoperation has contaminated the core. It is also an object of thisinvention to employ a radioactive chemical-tracer `which can bequantitatively determined if complete flushing of the core by the mudfiltrate occurs. These and other objects will become more apparentfromthe following detailed description of 'this invention.

Figure l illustrates a typical radioactivityprole of a core recovered byemploying an embodiment ofthe core sampling technique of this invention.

Figures 2, 3, and 4 show radioactivityproles of oil Weil corespenetrated to various depths by radioactive solutions.

In act-ual practice, core samples uncontaminated 'by `the drillingmudfiltrate are Vvery ditlicult to obtain with consistency. VBecausevarying degrees'of iiushing by the drilling mud filtrate occur duringthe coring operation,

the `extent of` flushing must be determined in order to properlyevaluate the iluid saturations in the core as obtained from coreanalysis.

To diiiferentiate between the total uid content of the corefresultingfrom the invasion of drilling mud filtrate and naturally occurringfluids, various techniques have beendeveloped `using chemical reagentsto trace the intrusionfof the drilling mud rltraite. Because it isgenerallypreferred that Water-base muds be used instead of an oil-basemud in coring techniques, a Water ltrate is usually `incorporated `inthe core sample. These chemical techniques have required quantitativeanalytical methods for determining the extent of contamination of the`core sample by drilling mud filtrate. To carry outthis chemicalanalysis, it is necessary to destroy the core sample, thereby preventingfurther use of the core for other analyses. Although such `atime-consuming'analysis may permit an evaluation ofthe fluid contents,it does not indicate fthe portions of the core which have been`invaded-bythe drilling mud ltrate. `According to this invention ithasbeen found that the degree ofV invasion :of the i. core `sample bydrilling mud filtrate `can be eX- peditiously carried out in anon-destructive manner by utilizing radioactive `chemical-tracertechniques. In carv,ryingtout thelcore-sampling technique of thisinvention, .afsmallramount of a radioactive lmaterial is added to the-drillingmud employed inconventional coring operations.

Thereafter, the cores which are obtained are subjected to thelscrutinyfof radiation detectors fordetermining the extent. to which thedrilling mud filtrate has invaded the core sample surface.

Radioactive-materials emit radiations ot three primary types:

('l) 4Alpha particles: These are identical to yhelium `nuclei-andareusually emitted from radio-isotopes of the -heavier elements. Theypossess very little penetrating `power but produce intense ionizationalong their` paths.

(2) wletaparticles: These are negative electrons .andi

are emitted from a variety of radioisotopes. They exhibit greaterpenetration and lower ionization intensity than alpha particles.

(.3) Gamma rays: These are electromagnetic rays similar to X-rays, butusually of higher energy. They exhibit much greater penetration andlower ionization intensity than beta particles.

`Detection and measurement of these radiations are usually accomplishedby `sensitive Aelectronic equipment Vcomprising a Geiger tube,scintillation counter, orother suitable detector, and a sealer orratemeter to indicate ythe detector output. In tests made for tracingmud filtrates indrill cores, a thin-window Geiger tube and a-scalerfhave been used to measure beta radiation from a beta-gammaemitter used as a tracer.

-Thechoice of a beta, beta-gamma, or gamma-emitting isotopefor a traceris largely dependent upon the nature ofthe measurements to be made andthe available instrumentation. A system of measurement is preferredwhich requiresthe least concentration of radioactivity andutilizesreadily obtained or modified equipment. Final selection of aspecific isotope is governed by half-life, possible-biological hazards,physical `and chemical stability, and availability of usable chemicalforms.

Alpha-emitters as a Whole are considered extremely hazardous, areavailable in verylimited quantities, and present practical problems inmeasurement because of their veryflimited penetration. Accordingly,alpha emitters would not be preferred as tracers in the process of thisinvention.

university laboratories has shown that radioisotopes can be handled verysafely. A few simple precautions are adequate for protection of theworker from harmful radiation exposure in radiotracer work. Simplemonitoring procedures and devices such as pocket dosimeters and filmbadges are employed to assure that radiation exposures are within thelimits recommended by the National Committee on Radiation Protection,which is sponsored by The National Bureau of Standards.

Many radioisotopes are available for use as tracers.

A partial list includes:

Beta-emitters: yttrium-90, phosphorous-32, bismuth- 210, calcium-45,sulfur-35, carbon-14, etc.

Beta-gamma-emitters: cadmium-115, gold-198 and l99, molybdenum-99,antimony-l22, iodine-131, barium- 140, iron-59, zirconium-95, cobalt-60,bromine-82, silverlll, etc.

Gamma-emitters: chromium-l,` tin-113, barium-131, mercury-197, etc.

All of these isotopes are available through the facilities of the AEC,usually in the form of chlorides or nitrates in solution. A tracer inany other chemical form desired can usually be prepared from thesematerials.

For tracing aqueous drilling fluids, the chemical forms in which theisotopes are received from the supplier are usually preferred since nofurther chemical preparation is required. However, their physical andchemical stability in the system to be investigated should be checkedbefore use. Although not ordinarily used as a drilling fluid in coringoperations, oil-base or oil-emulsion drilling muds can be employed, ifdesired, for specific applications. Oil-soluble tracers incorporatingthe radioisotope to be used are prepared by organic synthesis, nuclearrecoil reactions, or other suitable processes. Examples of suchmaterials are benzene (C-l4), iodobenzene (I-l31), trieresyl phosphate(P32), carbon disulfide (C-l4 or-S- 35), iron (Fe-59) or cobalt (C0-60)naphthenates, etc.

In measuring the radioactivity of the core sample, conventionalradiation equipment comprising Geiger-Mueller counters, scintillationcounters, or the like, can be used. Because of their beta-countingefficiency, thin-window Geiger-Mueller counters are preferred for use asradiation detectors. Other counting methods can be utilized fordetermining the specific activity of the core sample being analyzed.Various methods for measuring radioactivity and detecting and measuringequipment are comprehensively considered in such standard texts as:

Taylor: Measurement of Radioisotopes, Wiley (1951). Korpf: Electron andNuclear Counters, Van Nostrand (1946), and Sharpe: Nuclear RadiationDetectors, Wiley (1955).

In obtaining the core samples employed in this invention, conventionalrotary drilling equipment employing rotary core-barrel assemblies areutilized. Because the specific manipulative techniques employed inobtaining the core cuttings are not within the scope of this invention,reference is made to Subsurface Geologic Methods, Le Roy, ColoradoSchool of Mines at page 609 et seq.,

VPetroleum Production Engineering, Uren, 3rd edition,

McGraw-Hill, for coring techniques.

In conducting the coring operation, water-base drilling iluids arepreferably employed. The water-base fluids generally comprise asuspension or gel of bentonite in water weighted by 325 mesh bariumsulfate. The instability of these fluids to salt and calcium can beovercome by the addition of such materials as starch, tannin, gums orpolyphosphates. Oil-base type drilling fluids, if used, generally areformulated from fuel oils which have been gelled by the addition ofblown asphalts, lime or various other types of soaps. Calcium carbonateor weakly hydrophilic oxides, such as magnitite or litharge, can beemployed as weighting materials. Drilling fluids of the emulsion typeare essentially water-base muds to which an emulsifying agent and oilhave been added. Proper selection and composition of the drilling fluidemployed will depend upon the characteristics desired. For a morecomprehensive discussion of drilling uids reference is made toComposition and Properties of Oil Well Drilling Fluids, Rogers, Gulf.The amount of radioactive tracer which is incorporated in the drillingfluid will depend upon the level determined by (a) radioactive decaybefore measurement, (b) efficiency of measurement, (c) formationporosity, if known. In general, an amount sufficient to yield aradioactivity of about l to 10 mc. per barrel of drilling duid can besafely employed. In using amounts of radioactive materials which willgive activities in excess of this, extreme care must'be taken to avoidexposure of operating personnel to excess dosages.

In carrying ont the process of this invention, a desired amount ofradioactive tracer is incorporated in the drilling mud to be employed inthe coring operation by simple admixing during the formulation of thefluid, or in the course of the coring operation. By means ofconventional coring techniques, core samples are collected. Although thecore samples can be evaluated in the field, it is preferred that they bereturned to the laboratory for analysis. Shipment can be made bywrapping the samples in aluminum, lead foil or other protective wrappingwhich will effectively prevent any radiation from escaping from thepackage, and placing the wrapped cores in a container which is thensealed. The smaller the core diameter the greater the chance forconamination throughout the core. Therefore, it is desirable to cut acore that has a diameter that would not permit complete penetration. Theminimum diameter for reliable results would be 3"-412. Cores larger than41/2" can be obtained, but are not usually cut. Cores below 3 indiameter can be used, but the results are not as reliable. In preparingthe cores for evaluation, the cores obtained are transversely fracturedwith respect to the longitudinal axis of the core to provide a cleanface. The flat, transverse face is obtained in one manner by laying thecore on a solid surface and placing a stone-masons chiselperpendicularly to the axis of the core. By hitting the chisel with ahammer the core will usually break perpendicular to the axis or alongthe bedding plane. A fresh break about 1 to 2 in from either end of thecore is then used for scanning with a suitable radiation detector. Byconducting scauning measurements along various diameters, aradioactivity profile of the core, such as that shown in Figure 1, canbe obtained. The use of this method thereby permitsv a rapiddetermination of the extent to which contaminant has penetrated, andpermits removal of the unaffected portion of the core for laboratoryoiland water-saturation determinations. In the event that the core iscompletely contaminated, the amount of contaminating water in the corecan be quantitatively determined by the measurement of the radioactivityof the core.

In essence, this complementary, analytical procedure involves taking asection of the completely contaminated core, and comminuting it toproduce small granular pieces. The pieces are leached with distilledwater and the radioactive tracer recovered in the leachate. When aradioactive iodide is employed as the tracer material it is precipitatedfrom the leachate using a solution of silver nitrate in accordance withconventional quantitative analytical techniques. The precipitate iscollected on a filter paper and its radioactivity determined. Bycomparing the counting rate with a calibration curve which shows theamount of radioactivity in the core, the amount of contamination by thedrilling mud filtrate can be determined. After the total watersaturation has been determined by conventional techniques, the connatewater saturation may be calculated by subtracting the amount ofcontaminating water from the total water value. If the core is indicatedto be completely contaminated it is, therefore, known that thesaturations determined by "gs-frenar Aioiverltional' core analysistechniques are n'o't representative of reservoir uid saturation. Cautionshould then be usedbefore completing the'welL If'the "centerseciti'onsshown no'activitythen thecore analysis results are more representativeof 'saturation in the Ireservoir. KYThese data can then be used withmore reliability when 'completing or evaluating the oil` reservoir. Y

'Ihe subject invention is illustratedfby `the` following "specicexamples. 'Initial laboratory` tests were made to evaluate the4performance of Athe counting apparatus and determine lthe detectabilityof the radioactive tracer in l ment in `cores saturated with radioactivesolution.

The primary instrumentation used was a' thinwindow Geiger counterconnected to a conventional sealer which .provided a regulated 4D.'C.high voltage and totalized .pulses from `the counter tube. The nGeigertube was shielded with l of lead to reduce background radiation.

l" Lucite -disc with `a cur-ved slit aperture GA wide x 11/s-chordlength, radiusof curvature 11/z) covered the tube window and defined thearea of measurement. A number of natural Berea sandstone radial cores, 3inches in diameter, were vprepared with drilled centerholes ranging indiameter from Vs inch to 1 inch. The cores were evacuated and saturatedwith aqueous solutions containing 15 andlS()` microcuries of iodine-131per gallon and 0.01 gram of ordinary sodium iodide per `gallon -as acarrier. Unsaturated sandstone plugs were placed in the center holesandulead wool was packed around the plugs so that no void space existedAbetween the plug land the core. The-corewas Arplacedvin a holder andthe core surface -mmediatelyscanned with a Geiger counter. The core wasstored "in a Adesiccator and "scanned periodicallythereafterforI-'several days. `A numjber of cores wereVstudiedAin-thismanner.

A v representative graphical presentation of the results of `this workis shown in Figure 2 where it is seen that a definite line ofdemarcation can be detected employing a conventional radiation detector.

Cores obtained from producing formations would ordinarily not becompletely saturated with the radioactive mud filtrate but would alsocontain connate water and oil. The presence of other fluids in the corealong with the contaminating fluid could affect the counting rate on thecore surface. Three radial cores, 3% inches in diameter, from variousproducing horizons were used in another series of tests. In these teststhe cores were saturated with oil and water to simulate the fluiddistribution in the cores in the reservoir prior to coring. These coreshad a 1li-inch hole drilled vertically through the center of the core.The cores were extracted and porosity and permeability measurementsmade, after which the cores were saturated with a 5 percent brinesolution. Using the restored state method the cores were then reduced toequilibrium water saturation by injection of Soltrol, a proprietarypetroleum naphtha marketed by Phillips Petroleum Co. An aqueous solutioncontaining 150 microcuries of iodine-131 per gallon was then injectedinto each core to simulate fluid saturaltion after coring. The core wasysplit in half (parallel to the bedding plane) and visual observationfor the depth of penetration of the radioactive solution was made.Counting measurements for radioactivity were made on .a fresh coresurface at different distances from the edge 'of the core. The coreswere stored in a desiccator and `.scanned with the Geiger counterperiodically thereafter .for two weeks.

The activity in one of the three test cores, core #l `which `is shown,graphically `tween l1/2 and 3%: inch from the edge of the core.

lfrom the center to '1%6 Ainches from the edgeof "the core. A scan ofthe core surface showed the interface of activity to`be between i1 and1% inches from the outer edge of thecore, in satisfactory agreementwiththe calculated value. The coresur'face was scanned every day for-sevendays after which timethe'interface of activity was still in the sameposition. The counting rate on the core surface did not change at anyposition over the seven days of measurement, as would `be expected fromiodine radioactivity decay. Continued evaporation at the core surfaceapparentlyconcentrated activity at the surface continuously andsufficiently to maintain the original counting rate.

The observed (visual) depth of penetration of 'the radioactivity in coreNo. 2 is given in Figure 4. The

core surface was first scanned Valong radius 1 and the counting rateindicated the radioactive front to 'be be- The observed depth ofactivity was at 3A inch in from the edge of the core and in fairagreement. with the value obtained from scanning. The coresurface alongradius Vl was scanned 6 days `later andthe position of the activityfront was'found to`be unchanged. The lower overall counting rate on thesixth day was Vcaused by decay of activity. Results obtained by scanningthe corealong radius 2 showed that the activity front was very close,within Mt inch, tothe core edge. Visual observation of the rcore surfaceverified the location ofactivity.

.Following the counting measurements, chips from the active center.section and the indicated non-active sec- `tions were evaluated,qualitatively, by the precipitation .radioactive material present intheprecipitate.

From .these results `it is seen that (l) an interface of activity finoil-saturated cores can be located to within 1A of an inch in scanningthe core surface with a Geiger counter, and .(2) the radioactiveinterface as` indicated by surface activity content remains essentiallystationary for several days; it does not diffuse through the core.Although it might be suggested from the visual profile that a visualobservation technique could be used, dyes or fluorescent materials,however, would not be operative since these materials are usually highlysurface-active and the clay material in the core would adsorb thematerials. Accordingly the dye would not be carried to the depth ofpenetration of the filtrate due to adsorption.

By means of the instant invention, mud filtrate invasion ofA geologicalcore samples can be accurately, easily, and economically measured toprovide reliable information about depth of penetration of thecontaminant. Radioactive chemical-tracer materials with a half-lifesufficiently long to permit radioactivity measurements to beconveniently made, but sho1t enough to facilitate disposal anddecontamination, are readily available. The concentration of radioactivematerial added to the drilling mud is sufficiently low to avoid anyserious hazard in connection with the operating personnel. Thenon-destructive nature of the subject technique provides an advantagenot available in other prior art, core-analysis techniques. It isevident that departures from the specific illustrative examples of thisinvention can be made without going beyond the scope of this invention.Such modifications are apparent to those skilled in this art.Accordingly, this invention is defined ini the following claims as:

l. A process for measuring the invasion and depth of a Huid-soluble,radioactive tracer material in the drilling Afluid in an amount suicientto impart radioactivity to said iluid used in said coring, collecting atubular core vsample during said coring, transversely fracturing saidsample with respect to the longitudinal axis of said core surface, anddetermining the radioactivity prole of said vsurface by scanning saidsurface with a suitable radioactivity counter.

3. A process for measuring the invasion and depth of penetration of mudiiltrate in geological core samples obtained by corinrg subterraneangeological strata ernploying an aqueous drilling fluid, which comprisesincorporating a Water-soluble radioactivity tracer material in thedrilling iluid in an amount suiiicient to yield a radioactivity of about1-1O millicuries per barrel of drilling fluid, collecting a tubular coresample during said coring, transversely fracturing said sample withrespect to the longitudinal axis of said core to provide a clean,substantially iiat surface, and determining the radioactivity profile ofsaid surface by scanning said surface with a suitable radioactivitycounter.

4. A process in accordance with cla'un 3 in which radioactive iodine isemployed as the tracer material.

5. A process for measuring the invasion and depth of penetration of mudfiltrate in geological core samples obtained by coring subterraneangeological strata employ-ing an aqueous drilling fluid, which comprisesincorporating a water-soluble, radioactive tracer material in thedrilling Huid used in said coring, collecting a tubular core sampleduring said coring, transversely fracturing said sample with respect tothe longitudinal axis .8 of said core to provide a'clean, substantiallytlat surface, and determining the radioactivity prole of said surface byradially scanning said surface'lwith a suitable radioactivity counter.6. A process for' measuring the invasion and depth of penetrationof mudfiltrate in geological core samples obtained by coring subterraneangeological strata employing an aqueous drilling iiuid, which comprisesi-Ycorporating a water-soluble radioactivity tracer material .1n thedrilling fluid in anamount suicient toyield a radioactivity offabout1-10 millicuries per barrel of drilling fluid, collecting a tubular coresample during said corng, transversely fracturing said sample withrespect to the longitudinal axis of said core to provide k'a clean,substantially at surface, and determining the radioactivity of saidsurfacev by radially scanning said surface with a suitable radioactivitycounter. 4

7. A process in accordance with 'claim `6 in which radioactive iodine isemployed as the tracer material.

8. A' process for measuring the invasion and depth of penetration ofrnud ltrate in geological core samples obtained by coring subterraneangeological strata employing an aqueous drilling uid, which comprisesincorporating a water-soluble radioactive tracer material consistingessentially of I-131 in the drilling fluid in an amount suicient toyield 1-10 millicuries per barrel of drilling fluid used in said coring,collecting a tubular sample during said coring, transversely fracturingsaid sample with respect to the longitudinal axis of said core by layinga knife edge transverse to longitudinal axis of the core intermediatethe extremities thereof and striking a sharp blow against said knifeedge thereby fracturing said core and providing a substantially mud-freeflat surface, and determining the radioactivity of said surface byradially scanning it with a suitable radioactivity counter.

References Cited in the file of this patent UNITED STATES PATENTS2,110,310 Shayes et al. Mar. 8, 1938 2,544,412 Bird Mar. 6, 19512,840,717 De Witte June 24, 1 958

1. A PROCESS FOR MEASURING THE INVASION AND DEPTH OF PENETRATION OF MUDFILTRATE IN GEOLOGICAL CORE SAMPLES OBTAINED BY CORING SUBTERRANEANGEOLIGICAL STRATA EMPLOYING A DRILLING FLUID, WHICH COMPRISESINCORPORATING A FLUID-SOLUBLE, RADIOACTIVE TRACER MATERIAL IN THEDRILLING FLUID IN AN AMOUNT SUFFICIENT TO IMPART RADIOACTIVITY TO SAIDFLUID USED IN SAID CORING, COLLECTING A TUBULAR CORE SAMPLE WITH RESPECTTO THE LONGITUDINAL AXIS OF SAID CORESAMPLE WITH RESPECT TO THELONGITUDINAL AXIS OF SAID CORETO PROVIDE A SUBSTANTIALLY MUD-FREE,FLATSURFACE, AND DESCANNING SAID SURFACE WITH A SUITABLE RADIOACTIVITYCOUNTER.